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Abstract

We demonstrate highly efficient Raman distributed feedback (DFB) fibre lasers for the first time with up to 1.6W of continuous wave (CW) output power. The DFB Bragg gratings are written directly into two types of commercially available passive germano-silica fibres. Two lasers of 30cm length are pumped with up to 15W of CW power at 1068nm. The threshold power is ~2W for a Raman-DFB (R-DFB) laser written in standard low-NA fibre, and only ~1W for a laser written in a high-NA fibre, both of which oscillate in a narrow linewidth of <0.01nm at ~1117nm and ~1109nm, respectively. The slope efficiencies are ~74% and ~93% with respect to absorbed pump power in the low-NA fibre and high-NA fibre respectively. Such high conversion efficiency suggests that very little energy is lost in the form of heat through inefficient energy transfer. Our results are supported by numerical simulations, and furthermore open up for the possibility of having narrow linewidth all-fibre laser sources in wavelength bands not traditionally covered by rare-earth doped silica fibres. Simulations also imply that this technology has the potential to produce even shorter R-DFB laser devices at the centimetre-level and with mW-level thresholds, if Bragg gratings formed in fibre materials with higher intrinsic Raman gain coefficient than silica are used. These materials include for example tellurite or chalcogenide glasses. Using glasses like these would also open up the possibility of having narrow linewidth fibre sources with DFB laser oscillating much further into the IR than what currently is possible with rare-earth doped silica glasses.

DFB output spectra just below and just above threshold of R-DFB1 with 0.1nm RBW (a) and 0.01nm RBW (b), and R-DFB2 with 0.1nm RBW (c) and 0.01nm RBW (d). The powers shown in the figure are the incident pump power.